The adenovirus-associated viruses (AAV) are small, defective, human parvoviruses which require co-infection with a helper adenovirus to multiply. The AAV virion contains three polypeptides and their combined molecular weight exceeds the coding capacity of the genome. This investigation deals with the mechanism by which AAV produces an amount of protein in apparent excess of its genetic information. Some of the other parvoviruses, e.g., rat virus, H-l virus, and hemadsorbing enteric virus (HADEN), also contain three polypeptides, with a molecular weight greater than that predicted on the basis of genome size. AAV is used as a model to predict a possible mechanism of structural protein biosynthesis in the parvovirus group. Our experiments involve techniques determining common antigens and amino acid sequences between the three polypeptides. We are searching for a precursor polypeptide compatible with the genome size which may be cleaved into the virion structural components. We have produced antisera against each of the sodium dodecyl sulfate (SDS) - treated structural polypeptides of AAV. These antisera are specific for antigens on the polypeptides. We are using these sera in the indirect radio-immuno-precipitation test to detect crossreactivity between the polypeptides and to study the crossreactions between homologous peptides among the various AAV serotypes. Information on the molecular relationships of these polypeptides is being gained by their electrophoretic patterns on polyacrylamide gel when labeled with single basic, neutral and acidic amino acids by isoelectrofocusing, and by peptide mapping. Pulse labeling experiments are to be performed in attempting to identify a precursor protein and showing that the radioactive amino acid residues appear in the final polypeptides. We are purifying empty AAV particles by sedimentation through linear sucrose gradients to determine what polypeptides are present. Antisera prepared against the three HADEN virus polypeptides are to be used in immunofluorescence tests and for studies on interpeptide antigen sharing.